U.S. Pat. No. 8,390,928 B2 discloses a phase plate consisting of a pair of glass types which are selected such that, at a particular thickness for each glass in the pair, the phase difference in propagation of light through one glass of the pair is exactly one half wavelength relative to the other glass of the pair, for at least three different wavelengths. Where one glass of the pair is called A and has a thickness TA and the other class is B and has a thickness TB, eight squares of glass are made, two squares “A” of glass A of thickness TA, two squares “A*” of glass A of thickness TA/2, two squares “B” of glass b of thickness TB and two squares “B*” of glass B of thickness TB/2. Then the four sandwiches of the squares are arranged in a 2×2 quadrant square with a sandwich of glass A and A* in the upper left hand quadrant, of glass B and B* in the upper right hand quadrant, of glass B and A* in the lower right hand quadrant and of glass A and B* in the lower left hand quadrant. A light beam passing through this array, when focused to a focus point, will have a zero intensity central point at the focus point for each of the three wavelengths for which the half wavelength difference is exact and close to zero intensity for the intermediate wavelengths. This known phase plate is not configured to let a beam of light of another wavelength pass through unaffected such that, when focused to a focus point, it has a local intensity maximum at the focus point. Further, the sandwiches of this known phase plate are of different thicknesses.
International patent application publication WO 2011/086519 A1 discloses a phase modifying member inserted in a common optical path of both an excitation and a depletion beam of an STED fluorescence light microscope. The phase modifying member leaves the wavefronts of the excitation beam unchanged but changes the wavefronts of the stimulation or depletion beam so as to create an undepleted region of interest when focusing the depletion beam to a focus point. The phase modifying member has a surface with a plurality of regions of different heights along the common optical path. The heights are selected such that they change the phase of the excitation beam in modulus of 2 times π, whereas the phase modifying member modifies the phase of the depletion beam such that the modified phase as a function of the azimuthal angle around the common optical phase is approximately equal to the azimuthal angle. The phase modifying member has to be manufactured at a very high precision with regard to its regions of different heights to modify the phase of the depletion beam and to leave the wavefronts of the excitation beams substantially unchanged. Further, the optical element is only operating for two different wavelengths of the excitation beam on the one hand and the depletion beam on the other hand for which the phase modifying member is designed.
U.S. Pat. No. 8,755,116 B2 discloses a wavelength sensitive phase filter consisting of two pairs of optical wedges. The two pairs are arranged on opposite sides of a main axis; and the optical wedges are made of two different materials. The outer surfaces of the pairs of optical wedges are plane-parallel. For one light component of a first wavelength, there is a refractive index difference Δn between the materials of the wedges. As a result, two parallel linear phase ramps with gradients in opposite senses are applied to the wavefronts of the one light component, as it passes through the phase filter. In contrast, the refractive indices of the materials of the wedges are identical for another light components of a second wavelength. As a result, the wavefronts of the other light component are not affected as they pass through the phase filter.
U.S. Pat. No. 8,755,116 B2 further discloses a modification of the above phase filter which is based on three different materials. Optical wedges of these three different materials are arranged with two different wedge angles, one angle between the first and second and a different angle between the second and third material. The second material junction, and possibly even more materials with further wedge angles, make(s) it possible to configure the profile of the overall dispersion curve in order to produce local plateaus in the dispersion curve over which the phase shift, normalized with respect to the wavelength, of the one light component does not change with the wavelength. In addition, three or more materials can be used to ensure that the wavefront of the other light component remains constant as it passes through the phase filter, if this is not possible using only two materials.
With regard to the phase filter with two pairs of optical wedges of two materials, U.S. Pat. No. 8,755,116 B2 incorporates a further document by reference: D. Wildanger et al., “A STED Microscope Aligned by Design”, Opt. Exp. 17 (18), 16100-16110 (2009). This document states that an optimal phase filter for focal plane resolution enhancement in STED fluorescence light microscopy is a vortex which produces a linear increase in phase from 0 to 2π with increasing azimuthal angle about the beam axis of the STED light beam. Upon focusing, a circularly polarized STED light beam prepared in this way destructively interferes in the focal center and generates a zero-intensity minimum which is surrounded by a torus of high intensity. Besides optical wedges in a side by side arrangement producing two anti-parallel phase gradients, such a vortex phase filter may also be approximated by optical flats of constant phase. A phase filter consisting of six optical flats arranged around the beam axis already approximates the vortex well. For only modifying the wavefronts of the STED light beam but not affecting the wavefronts of an excitation light beam passing through the same phase filter, each of the six optical flats is made of two optical flats of materials whose refractive indices are equal at the excitation wavelength but notably differ at the STED wavelength. Particularly, the two materials are selected such that their refractive index difference is more than 5×10−4 or about 10−3 at the STED wavelength. With equal thicknesses of all pairs of optical flats stacked along the beam axis, the phase shifts induced at the excitation wavelength are the same for all pairs such that the wavefronts of the excitation light beam are not deformed. The phase shift induced at the STED wavelength is a function of the diffractive index difference between the two optical flats of each pair at the STED wavelength and of the thicknesses of the two optical flats in each pair.
US patent application publication US 2011/0310475 A1 discloses a microscope forming a beam spot in a desired shape in a focal plane. The microscope comprises a modulation optical element having a plurality of regions for spatial modulation of illumination light, and an adjustment element for adjusting an optical property of the illumination light modulated by the modulation optical element. The optical element may be a phase plate having a plurality of regions of different phase shifts for erasing light such that the erasing light once focused comprises a zero point in the focus. In the different regions, the phase plate of the known microscope is coated with an optical film being an optical multilayer. Different spatial arrangements of these regions and different relative phase shifts are disclosed to achieve the desired result of the zero point in the focus.
European patent application publication EP 1 622 137 A1 discloses an optical diffraction element to be disposed in an optical path through which a plurality of light beams of different wavelength travel. The optical diffraction element has a periodic structure which, when a first light beam having a first wavelength is in a linearly polarized state polarized in a first direction, allows the first light beam to be substantially completely transmitted thereto, but when the first light beam is in a linear polarized state polarized in a second direction perpendicular to the first direction, causes the first light beam to be substantially completely diffracted. In the periodic structure of the optical diffraction element, two regions are alternately arranged along an in-plane direction. This periodic structure constitutes a grating pattern for diffracting light. Each of the region is structured so that a plurality of medium layers having different refractive indices and/or thicknesses are stacked. When light is transmitted to the diffraction element, a phase difference occurs between the light transmitted through the regions, thus resulting in diffraction phenomenon. Due to an refractive index anisotropy of the layers, the phase difference depends on the polarization direction of linearly polarized light.
US patent application publication US 2013/0299716 A1 discloses an apparatus for generating a proton beam, which includes a laser system providing a laser pulse, a target generating a proton beam by using the laser pulse, and a phase conversion plate disposed between the laser system as a light source and the target to convert the lase pulse into a circularly polarized laser pulse having a spiral shape. The phase conversion plate may include a plurality of sectors that are divided from a circle in azimuthal direction. The sectors may include a crystal showing birefringence. The sectors may include quarter wave plates arranged in such a way as to convert the linearly polarized laser pulse into the circularly polarized laser pulse having the spiral shape. In addition, the circularly polarized laser pulse may be shifted by a phase of 2π radiance by the quarter wave plates. The quarter wave plates may generate a phase difference of an integer multiple of 2π radiance about the circularly polarized lase pulse.
In STED fluorescence light microscopy the STED light and/or the excitation light are preferably applied in very short pulses to limit a timewise overlap of these pulse with the emission of fluorescence light by a sample to be imaged. Commercial light sources, i.e. pulsed lasers for supplying excitation light and/or STED light in ultrashort pulses, display a rather wide emission wavelength bandwidth. A TI-sapphire-laser, for example, has an emission wavelength bandwidth of about 8 nm with ultrashort pulses. Known phase filters for shaping the STED light but for leaving the excitation light unaffected do not work over such a wavelength bandwidth of either the STED light or the excitation light. Particularly, the intensity of the focused shaped STED light does no longer go down to zero in the local intensity minimum at the focus point with such a wavelength bandwidth of the STED light.
There still is a need of a segmented phase plate which is capable of leaving wavefronts of light of a first wavelength unaffected whereas it purposefully shapes wavefronts of light of a second wavelength over an increased range of wavelengths of at least one of the first and second wavelengths.